The circulatory system and the heart's role in it ...

You may have often heard about the circulatory system in our body and wondered what exactly it does. Today, we explain how it operates in brief.

The circulatory system is responsible for the transport of water and dissolved materials throughout the body, including oxygen, carbon dioxide, nutrients, and waste. The circulatory system transports oxygen from the lungs and nutrients from the digestive tract to every cell in the body, allowing for the continuation of cell metabolism.

The circulatory system also transports the waste products of cell metabolism to the lungs and kidneys where they can be expelled from the body. Without this important function, toxic substances would quickly build up in the body. The human circulatory system is organised into two major circulations. Each has its own pump with both pumps being incorporated into a single organ the heart.

The two sides of the human heart are separated by partitions, the interatrial septum and the interventricular septum. Both septa are complete so that the two sides are anatomically and functionally separate pumping units. The right side of the heart pumps blood through the pulmonary circulation (the lungs) while the left side of the heart pumps blood through the systemic circulation (the body).

You must be already knowing that the human heart is a specialised, four-chambered muscle that maintains the blood flow in the circulatory system. It lies immediately behind the sternum, or breastbone, and between the lungs. The apex, or bottom of the heart, is tilted to the left side.

The heart is made up of two muscle masses. One of these forms the two atria (the upper chambers) of the heart, and the other forms the two ventricles (the lower chambers). Both atria contract or relax at the same time, as do both ventricles.

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How the heart pumps blood

The heart muscle pumps the blood through the body by means of rhythmical contractions (systole) and relaxations or dilations (diastole). The heart's left and right halves work almost synchronously.

When the ventricles contract (systole), the valves between the atria and the ventricles close as the result of increasing pressure, and the valves to the pulmonary artery and the aorta open. When the ventricles become flaccid during diastole, and the pressure decreases, the reverse process takes place.

From the right atrium the blood passes to the right ventricle through the tricuspid valve, which consists of three flaps (or cusps) of tissue. The tricuspid valve remains open during diastole, or ventricular filling. When the ventricle contracts, the valve closes, sealing the opening and preventing backflow into the right atrium. Five cords attached to small muscles, called papillary muscles, on the ventricles' inner surface prevent the valves' flaps from being forced backward.

From the right ventricle, blood is pumped through the pulmonary or semilunar valve, which has three half-moon-shaped flaps, into the pulmonary artery.

This valve prevents backflow from the artery into the right ventricle. From the pulmonary artery blood is pumped to the lungs where it releases carbon dioxide and picks up oxygen.

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The Systemic Circulation

From the lungs, the blood is returned to the heart through pulmonary veins, two from each lung. From the pulmonary veins, the blood enters the left atrium and then passes through the mitral valve to the left ventricle.

As the ventricles contract, the mitral valve prevents backflow of blood into the left atrium, and blood is driven through the aortic valve into the aorta, the major artery that supplies blood to the entire body. The aortic valve, like the pulmonary valve, has a semilunar shape.

The aorta has many branches, which carry the blood to various parts of the body. Each of these branches in turn has branches, and these branches divide, and so on, until there are literally millions of small blood vessels. The smallest of these on the arterial side of the circulation are called arterioles.

They contain a great deal of smooth muscle, and because of their ability to constrict or dilate, they play a major role in regulating blood flow through the tissues.

The blood passing through the arterioles passes through a bed of minute vessels called capillaries, which are a single cell thick. These capillaries are so small that the red blood cells must line up single file to pass through. The exchange of nutrients and waste products takes place between the capillary blood and the tissue fluids. The arterialised blood that enters the capillaries thus becomes venous blood as it passes through them.

The capillaries empty the venous blood into collecting tubes called venules, and these in turn empty into small veins, which empty into larger veins, and so on, until finally all the blood returns to the heart through two large veins, the superior and inferior vena cavae. These terminate in the right atrium, and the systemic circulation is complete.

A one-way flow of blood in this system is maintained by valves located, not only in the heart, but in the veins as well. Some veins also have semilunar valves and the pressure of contracting muscles against the veins works with the action of these valves to increase the venous return to the heart. This is the reason that exercise is so important for the circulation.

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Coronary arteries and veins

The coronary arteries supply blood to the heart muscle. These vessels originate from the aorta, immediately after the aortic valve, and branch out through the heart muscle. The coronary veins transport the deoxygenated blood from the heart muscle to the right atrium.

The heart's energy supply is almost completely dependent on these coronary vessels. When the coronary vessels become blocked, as in arteriosclerosis or hardening of the arteries, blood flow to the cardiac muscle is compromised.

This is when the common "bypass surgery" is performed where the coronary arteries are "bypassed" by replacing them with, for example, a vein from the leg. A "double bypass" is when two coronary arteries are bypassed. A "triple bypass" is when three are bypassed, etc.